Recently, transparent conducting thin films are used in various fields, such as optoelectronic fields, display fields and energy industrial fields using organic and inorganic materials. In the field of semiconductor light emitting devices including light emitting diodes and laser diodes, materials having superior electrical and optical characteristics must be used in order to promote carrier injection and current spreading and to facilitate emission of photons generated from an active layer of a semiconductor light emitting device.
Many domestic and foreign institutes related to group III nitride light emitting diodes (III nitride LEDs), which are spotlighted as next-generation light sources for illumination, have actively studied to develop transparent conducting thin films. As a result, recently, transparent conducting materials, such as well-known indium tin oxide (ITO) and doped zinc oxide (ZnO) containing various impurities, are directly used as electrodes for nitride-based LEDs.
Among transparent conducting oxides (TCO), indium oxide (In2O3) tin oxide (SnO2), cadmium oxide (CdO), zinc oxide (ZnO), and indium tin oxide (ITO) have been actively studied and developed. The above oxides have relatively low work function values and represent characteristics of suddenly lowering light transmittance at wavelength bands of a visible ray and an ultraviolet ray, so problems occur when the above oxides are used for transparent electrodes of the nitride LED. Problems of the above oxides, which are partially used for the nitride LED, are as follows.
First, since conventional TCO or transparent conducting nitride (TCN) has a work function value which is significantly lower than that of a p-type nitride-based cladding layer, if the TCO or TCN is used as a p-type ohmic contact layer, a high energy barrier is formed at an interfacial surface against the carrier flow, so hole injection is very difficult. For this reason, realizing an LED having high external quantum efficiency (EQE) is very difficult.
In addition, since conventional TCO or TCN does not flexibly match with the electric characteristics formed at the surface of an n-type nitride-based cladding layer, if the TCO or TCN is used as an n-type nitride-based Schottky or an ohmic contact electrode structure, the controlling and injecting of holes relative to the carrier flow may be difficult. For this reason, realizing a light receiving diode or an LED having high light receiving efficiency or high external quantum efficiency (EQE) is very difficult.
Second, conventional TCO or TCN represents low light transmittance against specific lights created in and output from the nitride-based LED. In detail, the TCO or TCN represents low light transmittance against light having a wavelength band equal to or lower than that of a blue light, so the TCO or TCN is not suitable for to an LED that emits a light having a short wavelength.
Third, since conventional TCO or TCN has a great light reflective index approximating to 2, emitting the light to an atmosphere through the TCO or TCN is very difficult.
Recently, electronic devices, such as transistors and photo-detectors, and optical devices, such as LEDs and laser diodes (LDs), have been widely commercialized by using nitride-based semiconductors. In order to realize optoelectronic devices having superior performance, the contact controlling technology capable of improving the interfacial characteristics between the III nitride-based semiconductor and the electrode is very important.
LEDs using a nitride-based semiconductor including indium nitride (InN), gallium nitride (GaN) and aluminum nitride (AlN) are classified into top-emission LEDs (TELEDs) and flip-chip LEDs (FCLEDs).
According to the currently available TELED, light generated from the TELED is output through a p-type ohmic contact layer that makes contact with a p-type nitride-based cladding layer. In contrast, in the case of the FCLED, which is fabricated as a large-size and large-capacity LED because heat dissipation thereof is easily achieved during the operation as compared with that of the TELED, the light created from an active layer is emitted through a transparent sapphire substrate by using a high reflective p-type ohmic contact layer.
Since the p-type nitride-based cladding layer has a low hole-density, the LED employing the III nitride-based semiconductor may not easily transport holes, which are p-type carriers, in various directions at the p-type nitride-based cladding layer. Thus, in order to obtain the optoelectronic devices having superior performance using the p-type nitride-based cladding layer, a high-quality p-type ohmic contact layer having superior current spreading characteristics is essentially necessary.
In other words, in order to realize the high-quality next-generation LED by using the III nitride-based semiconductor, a p-type ohmic contact electrode structure capable of improving current spreading in the lateral direction and hole injecting in the vertical direction and having superior optical characteristics (light transmittance or light reflectance) for the visible ray and light having a short wavelength band must be developed.
The p-type ohmic contact layer of the TELED, which is extensively used in these days, includes oxidized nickel-gold (Ni—Au) formed on an upper portion of the p-type nitride-based cladding layer. A thin nickel-gold (Ni—Au) layer is deposited on the upper portion of the p-type nitride-based cladding layer by using an E-beam evaporator, and then the thin nickel-gold (Ni—Au) layer is annealed in the oxygen (O2) atmosphere, thereby forming a semi-transparent ohmic contact layer having a low specific ohmic contact resistance value of about 10−3 Ωcm2 to 10−4 Ωcm2. The oxidized Ni—Au ohmic contact layer has low light transmittance of 75% or less in a wavelength band of blue light, which is below 460 nm, so the Ni—Au ohmic contact layer is not suitable for the p-type ohmic contact layer of the next-generation nitride-based LED. Due to the low light transmittance of the oxidized semi-transparent Ni—Au ohmic contact layer, nickel oxide (NiO), which is p-type semiconductor oxide, is created in the form of an island at a contact interfacial surface between gallium nitride (GaN) forming the p-type nitride-based cladding layer and nickel (Ni) forming the ohmic contact layer when the oxidized semi-transparent Ni—Au ohmic contact layer is annealed at the temperature of about 500° C. to about 600° C. in the oxygen (O2) atmosphere. In addition, gold (Au) is interposed between nickel oxide (NiO) distributed in the form of an island while covering the upper portion of nickel oxide (NiO). In particular, when the thin Ni—Au layer deposited on the p-type nitride-based cladding layer is annealed in the oxygen (O2) atmosphere, the nickel oxide (NiO) is formed. Such nickel oxide (NiO) may reduce Schottky barrier height and width (SBH and SBW) formed between gallium nitride (GaN) and an electrode, so that carriers are easily fed into a device through the electrode when an external voltage is applied. The thin oxidized Ni—Au layer exhibits superior ohmic behavior that is a superior electric characteristic because nickel oxide (NiO) can reduce the SBH and SBW and Au component can improve the current spreading in the lateral direction.
In addition to the superior ohmic behavior mechanism of the thin Ni—Au layer, if the p-type nitride-based cladding layer is annealed after the thin Ni—Au layer has been deposited on the p-type nitride-based cladding layer, Mg—H intermetallic compounds that restrict the net effective hole concentration in the p-type nitride-based cladding layer can be removed. Thus, the net effective hole concentration can be increased to a level of above 1018/cm3 at the surface of the p-type nitride-based cladding layer through the reactivation process that increase concentration of magnesium dopants, so that a tunneling transport occurs between the p-type nitride-based cladding layer and the ohmic contact layer containing nickel oxide. Accordingly, the p-type nitride-based cladding layer exhibits the superior ohmic behavior with a low specific contact resistance value.
However, since the TELED employing the semi-transparent p-type ohmic contact electrode structure including the oxidized Ni—Au layer contains Au components that reduce the light transmittance by absorbing a great amount of light generated from the LED active layer, the TELED represents low EQE, so the TELED is not suitable for providing the large-size and large-capacity LED for illumination.
Recently, a document [T. Margalith et al., Appl. Phys. Lett. Vol 74. p 3930 (1999)] discloses the use of transparent conducting oxide, such as ITO, having superior light transmittance than that of the nickel-gold structure employed as a conventional p-type multi-layered ohmic contact layer, in order to solve the problems of the TELED and FCLED. A document (Solid-State Electronics vol. 47. p 849) shows that a TELED employing the ITO ohmic contact layer represents improved output power than that of a TELED employing the conventional nickel-gold structure.
However, although the ohmic contact layer employing the above ITO ohmic contact layer can increase the output power of the LED, the ohmic contact layer represents relatively higher operational voltage. This is because the ohmic contact layer has a relatively low work function value as compared with that of the p-type nitride-based semiconductor. For this reason, a high Schottky barrier is formed at the interfacial surface between the p-type nitride-based cladding layer and the ITO ohmic contact layer, so that carrier injection may not be readily achieved, thereby generating a great amount of heat and shortening the life span of the semiconductor device.
As mentioned above, if TCO such as ITO or ZnO is directly deposited on the p-type nitride-based cladding layer, the higher SBH and the wider SBW are formed so that the quality of the ohmic contact layer may be degraded. In order to solve this problem, a study group belong to GIST (Gwangju Institute of Science & Technology, Korea) recently discloses test results for high-quality ohmic contact layers including particles having a size of 100 nm or less, which are obtained by inserting a second TCO layer between a p-type nitride-based cladding layer and a first TCO layer and then annealing the resultant structure. The nano-particles created from the interfacial surface cause an electric field at the interfacial surface, so that the SBH and the SBW are reduced and the Schottky behavior of the TCO electrode is converted into the ohmic behavior by means of the electric field.
However, the high-transparent and high-quality p-type ohmic contact layer fabricated by using the above technologies and the vertical LED employing the same have limited light emitting areas and cause great heat dissipation during the operation, so the above p-type ohmic contact layer is not suitable for the next-generation light source for illumination.